Krypton

Krypton,  ₃₆Kr

A krypton-filled discharge tube glowing white
Spectral lines of krypton
General properties
Name, symbol	krypton, Kr
Appearance	colorless gas, exhibiting a whitish glow in a high electric field
Pronunciation	/ˈkrɪptɒn/ KRIP-ton
Krypton in the periodic table
Ar ↑ Kr ↓ Xe bromine ← krypton → rubidium
Atomic number (Z)	36
Group, block	group 18 (noble gases), p-block
Period	period 4
Element category	noble gas
Standard atomic weight (±) (Ar)	83.798(2)[1]
Electron configuration	[Ar] 3d^10 4s^2 4p^6
per shell	2, 8, 18, 8
Physical properties
Phase	gas
Melting point	115.78 K ​(−157.37 °C, ​−251.27 °F)
Boiling point	119.93 K ​(−153.415 °C, ​−244.147 °F)
Density at stp (0 °C and 101.325 kPa)	3.749 g/L
when liquid, at b.p.	2.413 g/cm^3[2]
Triple point	115.775 K, ​73.53 kPa[3][4]
Critical point	209.48 K, 5.525 MPa[4]
Heat of fusion	1.64 kJ/mol
Heat of vaporization	9.08 kJ/mol
Molar heat capacity	20.95[5] J/(mol·K)
vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 59 65 74 84 99 120
Atomic properties
Oxidation states	2, 1, 0 ​(rarely more than 0; unknown oxide)
Electronegativity	Pauling scale: 3.00
Ionization energies	1st: 1350.8 kJ/mol 2nd: 2350.4 kJ/mol 3rd: 3565 kJ/mol
Covalent radius	116±4 pm
Van der Waals radius	202 pm
Miscellanea
Crystal structure	​face-centered cubic (fcc)
Speed of sound	(gas, 23 °C) 220 m·s^−1 (liquid) 1120 m/s
Thermal conductivity	9.43×10^−3  W/(m·K)
Magnetic ordering	diamagnetic[6]
CAS Number	7439-90-9
History
Discovery and first isolation	William Ramsay and Morris Travers (1898)
Most stable isotopes of krypton
iso NA half-life DM DE (MeV) DP ^78Kr 0.35% >1.1×10^20 y (β^+β^+) 2.846 ^78Se ^79Kr syn 35.04 h ε – ^79Br β^+ 0.604 ^79Br γ 0.26, 0.39, 0.60 – ^80Kr 2.25% ^80Kr is stable with 44 neutrons ^81Kr trace 2.29×10^5 y ε – ^81Br γ 0.281 – ^82Kr 11.6% ^82Kr is stable with 46 neutrons ^83Kr 11.5% ^83Kr is stable with 47 neutrons ^84Kr 57.0% ^84Kr is stable with 48 neutrons ^85Kr syn 10.756 y β^− 0.687 ^85Rb ^86Kr 17.3% – (β^−β^−) 1.2556 ^86Sr
Decay modes in parentheses are predicted, but have not yet been observed

Krypton (from Greek: κρυπτός kryptos "the hidden one") is a chemical element with symbol Kr and atomic number 36. It is a member of group 18 (noble gases) elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquefied air, and is often used with other rare gases in fluorescent lamps. Krypton is inert for most practical purposes.

Krypton, like the other noble gases, can be used in lighting and photography. Krypton light has a large number of spectral lines, and krypton's high light output in plasmas allows it to play an important role in many high-powered gas lasers (krypton ion and excimer lasers), which pick out one of the many spectral lines to amplify. There is also a specific krypton fluoride laser. The high power and relative ease of operation of krypton discharge tubes caused (from 1960 to 1983) the official length of a meter to be defined in terms of the wavelength of the 605 nm (orange) spectral line of krypton-86.

History

Sir William Ramsay, the discoverer of Krypton

Krypton was discovered in Britain in 1898 by Sir William Ramsay, a Scottish chemist, and Morris Travers, an English chemist, in residue left from evaporating nearly all components of liquid air. Neon was discovered by a similar procedure by the same workers just a few weeks later.^[7] William Ramsay was awarded the 1904 Nobel Prize in Chemistry for discovery of a series of noble gases, including krypton.

In 1960, the International Conference on Weights and Measures defined the meter as 1,650,763.73 wavelengths of light emitted by the krypton-86 isotope.^[8][9] This agreement replaced the 1889 international prototype meter located in Paris, which was a metal bar made of a platinum-iridium alloy (one of a series of standard meter bars, originally constructed to be one ten-millionth of a quadrant of the Earth's polar circumference). This also obsoleted the 1927 definition of the ångström based on the red cadmium spectral line,^[10] replacing it with 1 Å = 10⁻¹⁰ m. The krypton-86 definition lasted until the October 1983 conference, which redefined the meter as the distance that light travels in a vacuum during 1/299,792,458 s.^[11][12][13]

Characteristics

Krypton is characterized by several sharp emission lines (spectral signatures) the strongest being green and yellow.^[14] It is one of the products of uranium fission.^[15] Solid krypton is white and has a face-centered cubic crystal structure, which is a common property of all noble gases (except helium, with a hexagonal close-packed crystal structure).

Isotopes

Naturally occurring krypton is made of six stable isotopes. In addition, about thirty unstable isotopes and isomers are known.^{[16] 81}Kr, the product of atmospheric reactions, is produced with the other naturally occurring isotopes of krypton. Being radioactive, it has a half-life of 230,000 years. Krypton is highly volatile when it is near surface waters but ⁸¹Kr has been used for dating old (50,000–800,000 years) groundwater.^[17]

⁸⁵Kr is an inert radioactive noble gas with a half-life of 10.76 years. It is produced by the fission of uranium and plutonium, such as in nuclear bomb testing and nuclear reactors. ⁸⁵Kr is released during the reprocessing of fuel rods from nuclear reactors. Concentrations at the North Pole are 30% higher than at the South Pole due to convective mixing.^[18]

Chemistry

Kr(H₂)₄ and H₂ solids formed in a diamond anvil cell.^[19]

Structure of Kr(H₂)₄. Krypton octahedra (green) are surrounded by randomly oriented hydrogen molecules.^[19]

Like the other noble gases, krypton is highly chemically unreactive. In fact, before the 1960s, no noble gas compounds had been discovered.^[20]

However, following the first successful synthesis of xenon compounds in 1962, synthesis of krypton difluoride (KrF
2) was reported in 1963. In the same year, KrF
4 was reported by Grosse, et al.,^[21] but was subsequently shown to be a mistaken identification.^[22] Under extreme conditions, krypton reacts with fluorine to form KrF₂ according to the following equation:

Kr + F₂ → KrF₂

Compounds with krypton bonded to atoms other than fluorine have also been discovered. There are also unverified reports of a barium salt of a krypton oxoacid.^[23] ArKr⁺ and KrH⁺ polyatomic ions have been investigated and there is evidence for KrXe or KrXe⁺.^[24]

The reaction of KrF
2 with B(OTeF
5)
3 produces an unstable compound, Kr(OTeF
5)
2, that contains a krypton-oxygen bond. A krypton-nitrogen bond is found in the cation [HC≡N–Kr–F]+
, produced by the reaction of KrF
2 with [HC≡NH]+
[AsF−
6] below −50 °C.^[25][26] HKrCN and HKrC≡CH (krypton hydride-cyanide and hydrokryptoacetylene) were reported to be stable up to 40 K.^[20]

Krypton hydride (Kr(H₂)₄) crystals can be grown at pressures above 5 GPa. They have a face-centered cubic structure where krypton octahedra are surrounded by randomly oriented hydrogen molecules.^[19]

Natural occurrence

The Earth has retained all of the noble gases that were present at its formation except for helium. Krypton's concentration in the atmosphere is about 1 ppm. It can be extracted from liquid air by fractional distillation.^[27] The amount of krypton in space is uncertain, as the amount is derived from the meteoric activity and that from solar winds. The first measurements suggest an overabundance of krypton in space.^[28]

Applications

Krypton gas discharge tube

Krypton's multiple emission lines make ionized krypton gas discharges appear whitish, which in turn makes krypton-based bulbs useful in photography as a brilliant white light source. Krypton is thus used in some types of photographic flashes used in high speed photography. Krypton gas is also combined with other gases to make luminous signs that glow with a bright greenish-yellow light.^[29]

Krypton mixes with argon as the fill gas of energy saving fluorescent lamps. This reduces their power consumption. Unfortunately this also reduces their light output and raises their cost.^[30] Krypton costs about 100 times as much as argon. Krypton (along with xenon) is also used to fill incandescent lamps to reduce filament evaporation and allow higher operating temperatures to be used for the filament.^[31] A brighter light results which contains more blue than conventional lamps.

Krypton's white discharge is often used to good effect in colored gas discharge tubes, which are then simply painted or stained in other ways to allow the desired color (for example, "neon" type advertising signs where the letters appear in differing colors are often entirely krypton-based). Krypton is also capable of much higher light power density than neon in the red spectral line region, and for this reason, red lasers for high-power laser light-shows are often krypton lasers with mirrors which select out the red spectral line for laser amplification and emission, rather than the more familiar helium-neon variety, which could never practically achieve the multi-watt red laser light outputs needed for this application.^[32]

Krypton has an important role in production and usage of the krypton fluoride laser. The laser has been important in the nuclear fusion energy research community in confinement experiments. The laser has high beam uniformity, short wavelength, and the ability to modify the spot size to track an imploding pellet.^[33]

In experimental particle physics, liquid krypton is used to construct quasi-homogeneous electromagnetic calorimeters. A notable example is the calorimeter of the NA48 experiment at CERN containing about 27 tonnes of liquid krypton. This usage is rare, since the cheaper liquid argon is typically used. The advantage of krypton over argon is a small Molière radius of 4.7 cm, which allows for excellent spatial resolution and low degree of overlapping. The other parameters relevant for calorimetry application are: radiation length of X₀=4.7 cm, density of 2.4 g/cm³.

The sealed spark gap assemblies contained in ignition exciters used in some older jet engines contain a very small amount of krypton-85 to obtain consistent ionization levels and uniform operation.

Krypton-83 has application in magnetic resonance imaging (MRI) for imaging airways. In particular, it may be used to distinguish between hydrophobic and hydrophilic surfaces containing an airway.^[34]

Although xenon has potential for use in computed tomography (CT) to assess regional ventilation, its anesthetic properties limit its fraction in the breathing gas to 35%. The use of a breathing mixture containing 30% xenon and 30% krypton is comparable in effectiveness for CT to a 40% xenon fraction, while avoiding the unwanted effects of a high fraction xenon gas.^[35]

Methods for the detection of concentrations of krypton-85 in the atmosphere have been used to detect suspected clandestine nuclear fuel reprocessing facilities in North Korea^[36] and Pakistan.^[37] These facilities were detected in the early 2000s and were believed to be producing weapons-grade plutonium.

Precautions

Krypton is considered to be a non-toxic asphyxiant.^[38] Krypton has a narcotic potency seven times greater than air, so breathing a gas containing 50% krypton and 50% air would cause narcosis similar to breathing air at four times atmospheric pressure. This would be comparable to scuba diving at a depth of 30 m (100 ft) (see nitrogen narcosis) and potentially could affect anyone breathing it. Nevertheless, that mixture would contain only 10% oxygen and hypoxia would be a greater concern.

References

1. Standard Atomic Weights 2013. Commission on Isotopic Abundances and Atomic Weights
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7. William Ramsay; Morris W. Travers (1898). "On a New Constituent of Atmospheric Air". Proceedings of the Royal Society of London 63 (1): 405–408. doi:10.1098/rspl.1898.0051.
8. "The BIPM and the evolution of the definition of the metre". Bureau International des Poids et Mesures. 2014-07-26. Retrieved 2014-09-24.
9. Penzes, William B. (2009-01-08). "Time Line for the Definition of the Meter". National Institute of Standards and Technology. Retrieved 2014-09-24.
10. Burdun, G. D. (1958). "On the new determination of the meter" (PDF). Measurement Techniques 1 (3): 259–264. doi:10.1007/BF00974680. (subscription required (help)).
11. Kimothi, Shri Krishna (2002). The uncertainty of measurements: physical and chemical metrology: impact and analysis. American Society for Quality. p. 122. ISBN 0-87389-535-5.
12. Gibbs, Philip (1997). "How is the speed of light measured?". Department of Mathematics, University of California. Retrieved 2007-03-19.
13. Unit of length (meter), NIST
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16. Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5.
17. Thonnard, Norbert; Larry D. MeKay; Theodore C. Labotka (2001-02-05). "Development of Laser-Based Resonance Ionization Techniques for 81-Kr and 85-Kr Measurements in the Geosciences" (PDF). University of Tennessee, Institute for Rare Isotope Measurements. pp. 4–7. Retrieved 2007-03-20.
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19. Kleppe, Annette K.; Amboage, Mónica; Jephcoat, Andrew P. (2014). "New high-pressure van der Waals compound Kr(H₂)₄ discovered in the krypton-hydrogen binary system". Scientific Reports 4. doi:10.1038/srep04989.
20. Bartlett, Neil (2003). "The Noble Gases". Chemical & Engineering News. Retrieved 2006-07-02.
21. Grosse, A. V.; Kirshenbaum, A. D.; Streng, A. G.; Streng, L. V. (1963). "Krypton Tetrafluoride: Preparation and Some Properties". Science 139 (3559): 1047–1048. Bibcode:1963Sci...139.1047G. doi:10.1126/science.139.3559.1047. PMID 17812982.
22. Prusakov, V. N.; Sokolov, V. B. (1971). "Krypton difluoride". Soviet Atomic Energy 31 (3): 990–999. doi:10.1007/BF01375764.
23. Streng, A.; Grosse, A. (1964). "Acid of Krypton and Its Barium Salt". Science 143 (3603): 242–243. Bibcode:1964Sci...143..242S. doi:10.1126/science.143.3603.242. PMID 17753149.
24. "Periodic Table of the Elements" (PDF). Los Alamos National Laboratory's Chemistry Division. pp. 100–101. Archived from the original (PDF) on November 25, 2006. Retrieved 2007-04-05.
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28. Cardelli, Jason A.; Meyer, David M. (1996). "The Abundance of Interstellar Krypton". The Astrophysical Journal Letters. The American Astronomical Society. pp. L57–L60. doi:10.1086/310513.
29. "Mercury in Lighting" (PDF). Cape Cod Cooperative Extension. Archived from the original (PDF) on September 29, 2007. Retrieved 2007-03-20.
30. Lighting: Full-Size Fluorescent Lamps. McGraw-Hill Companies, Inc. (2002)
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33. Sethian, J.; M. Friedman; M. Myers. "Krypton Fluoride Laser Development for Inertial Fusion Energy" (PDF). Plasma Physics Division, Naval Research Laboratory. pp. 1–8. Retrieved 2007-03-20.
34. Pavlovskaya, GE; Cleveland, ZI; Stupic, KF; Basaraba, RJ; et al. (2005). "Hyperpolarized krypton-83 as a contrast agent for magnetic resonance imaging". Proceedings of the National Academy of Sciences U.S.A. 102 (51): 18275–9. Bibcode:2005PNAS..10218275P. doi:10.1073/pnas.0509419102. PMC: 1317982. PMID 16344474.
35. Chon, D; Beck, KC; Simon, BA; Shikata, H; et al. (2007). "Effect of low-xenon and krypton supplementation on signal/noise of regional CT-based ventilation measurements". Journal of Applied Physiology 102 (4): 1535–44. doi:10.1152/japplphysiol.01235.2005. PMID 17122371.
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Further reading

• William P. Kirk "Krypton 85: a Review of the Literature and an Analysis of Radiation Hazards", Environmental Protection Agency, Office of Research and Monitoring, Washington (1972)

External links

• Krypton at The Periodic Table of Videos (University of Nottingham)
• Krypton Fluoride Lasers, Plasma Physics Division Naval Research Laboratory